This mystery can be found in the south of Wincestre (or the centre of its territory), in a small chapel southwest of The Old Minster. When you enter the chapel you’ll hear a man named Asser speaking frantically about a sign from god. When you speak with him, he will tell you that god’s light will show him his true path.
Eivor will comment about illuminating the lecterns to cease the prattling of Asser. To do this, look up to the northwest. You’ll see some windows on the left which are currently blocked. Go outside and climb the scaffolding on the northwestern side of the building. You’ll find some movable pillars of wood and stone blocking the two windows furthest to the right (southwest). Move these away from the window to allow the light to shine on the lecterns. Asser will see the light and claim that it is a sign from god. Soon after, he’ll be on his merry way and the mystery will be complete.